1. Field of the Invention
The present invention relates to a novel pigment which is employed in a coloring agent for a paint, a plastic, a cosmetic or the like, and a process for producing the same.
2. Description of the Related Art
A pearl mica pigment, for instance, has been known which is employed in a top coat paint for an automobile. This pearl mica pigment has a construction in which TiO.sub.2 is coated on the entire surface of mica, and it is used in a decorative painting which utilizes the interference colors resulting from the high refractive index of the TiO.sub.2.
The pearl mica pigment produces a variety of interference colors through the thickness variation of the TiO.sub.2 layer formed on the surface of the mica. When the TiO.sub.2 is coated in an amount of from 26 to 40% by weight with respect to the product, the pearl mica pigment produces gold. When the amount is from 40 to 50% by weight, it varies to produce red, blue and green as the thickness of the TiO.sub.2 layer increases. When the amount is from 50 to 60% by weight, it produces strong interference colors.
Although the pearl mica pigment has the pearly glossy effect and a variety of the interference colors, it always looks like white in the appearance. Hence, no pearl mica pigment has been available which produces a vivid color in the appearance.
Consequently, colored pigments such as iron oxide, iron blue, chromium oxide and carbon black have been added to the pearl mica pigment so as to attain a variety of colors in the appearance. However, when a plurality of these colored pigments are mixed to use, the inherent pearly glossy effect and interference colors of the pearl mica pigment are impaired, thereby deteriorating the decorativeness.
Therefore, the pearl mica pigment has been investigated in order to improve its decorativeness, and the following 3 representative methods have been reported so far: Japanese Unexamined Patent Publication (KOKAI) No. 19,666/1986, Japanese Unexamined Patent Publication (KOKAI) No. 164,653/1983, and Japanese Unexamined Patent Publication (KOKAI) No. 212,422/1984.
However, the method disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 19,666/1986 has a problem that it can produce only red and black pearl mica pigments. The method disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 164,653/1983 has a problem that it can produce only blue, bluish black, black and brownish black pearl mica pigments.
The method disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 212,422/1984 can color the pearl mica pigment in blue, green, gold, reddish purple. However, in this method, the pearl mica pigment should be heated and reduced in a hydrogen gas atmosphere at a high temperature of from 500.degree. to 1,000.degree. C. Since the thickness of the TiO.sub.2 layer is extremely thin, there is an anxiety for reducing the entire TiO.sub.2 layer when reducing at the high temperature of from 500.degree. to 1,000.degree. C. Accordingly, it is hard to control a component ratio of titanium with respect to oxygen in a particle diameter direction of the mica or the thickness-wise direction of the TiO.sub.2 layer. In addition, since the particles of the pearl mica pigments are sintered to aggregate and solidify at the high temperature, no fine powder pigment which is dispersed into its primary particles can be obtained. Hence, it is necessary to employ a special apparatus in order to maintain the powder state.
One might think of forming a titanium oxide layer on the mica particles themselves by sputtering titanium in an oxygen gas stream under decompression. However, the oxygen reacts with the titanium target to oxidize the target, and the sputterability through an oxygen gas is considerably small when compared with those through an argon gas and a helium gas. Hence, such a method is not practical. In addition, it is extremely hard to control the component ratio of titanium with respect to oxygen in the particle diameter direction of the mica or the thickness-wise direction of the TiO.sub.2 layer.
However, the pearl mica pigment is poor in the covering power and the metallic glossy effect. Accordingly, there have been proposed a variety of improved versions.
For example, Japanese Unexamined Patent Publication No. 161,055/1982 (KOKAI) discloses a pigment which includes a metallic coating layer formed on the entire surface of mica by plating.
Japanese Unexamined Patent Publication (KOKAI) No. 108,267/1989 which was applied by the present applicants discloses a pigment which includes metallic glossy dots formed on the surface of a pearl mica pigment in a scattering manner.
A pigment is commercially available from Shiseido Co., Ltd. under a trade name of "Infinite Color." The pigment includes a layer of oxides of titanium exhibiting low orders of oxidation states (hereinafter simply referred to as "low order oxides of titanium") which are formed on the entire surface of mica, and a TiO.sub.2 layer which is further formed on the entire surface of the layer of the low order oxides of titanium.
The pigment disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 161,055/1982 has a sharp metallic glossy effect because the entire surface of the mica is covered with the plated metal. However, it does not produce interference colors. Accordingly, it does not produce no deep effect.
The pigment disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 108,267/1989 operates through the reflection of the metal and scattering as well as the interference of the TiO.sub.2, and it produces both the interference colors and the metallic glossy effect. However, it produces the interference colors in an insufficient strength.
The commercially available "Infinite Color" produces the interference colors in a strength in a certain degree. However, it suffers from the insufficient metallic glossy effect and covering power.
Further, a metallic paint has been used widely in a top coat paint for an automobile. This metallic paint contains a foil-like aluminum powder. The metallic paint is favorable because of its high covering power. However, it is hard to make a metallic paint which produces a light color because the aluminum does not have such a high brightness.
Hence, a metallic paint comprising the pearl mica pigment which includes the TiO.sub.2 layer formed on the surface of the mica has been developed recently, and it is employed in practical applications. This metallic paint produces the pearly glossiness resulting from the mica as well as the interference colors resulting from the high refractive index of the TiO.sub.2. Even when the paint color is a light one, the metallic paint produces a glossy metallic effect in the appearance.
However, the metallic paint comprising the pearl mica pigment lacks the covering power, and it is often hard to carry out painting with the metallic paint under the two-coat-one-bake system, the current automobile painting process. In addition, the thus obtained paint film suffers from an insufficient metallic glossy effect when it is viewed in an oblique direction (i.e., in a shade direction).
Japanese Unexamined Patent Publication (KOKAI) No. 104,673/1989 discloses a metallic paint comprising a pigment in which a glossy metal is deposited on a part of a mica surface. This metallic paint, however, produces a metallic glossy effect only, and it is insufficient in the decorativeness.
The present applicants applied for a patent under Japanese Unexamined Patent Publication (KOKAI) No. 108,267/1989. The publication discloses a pigment which improves the above-mentioned shortcomings. The pigment includes metallic glossy dots which are formed on a surface of a pearl mica pigment in a scattering manner. A metallic paint comprising this pigment produces an enhanced metallic glossy effect when viewed in an oblique direction (i.e., in a shade direction), and it exhibits an improved covering power. However, the covering power and the interference colors of the metallic paint are still insufficient. Accordingly, there has been longed for a further improvement in the metallic paint.
Moreover, it is required for an automobile paint or the like, which is to be painted on an external metal surface particularly, to have an excellent weather resistance which enables the appearance of the paint not to change as long as a few years substantially though the paint is exposed to various weather conditions. This weather resistance is not only affected by the weather resistance of its resin components as a matrix, but also it is greatly affected by a pigment which are contained in the paint.
There have been proposed a variety of methods for improving the weather resistance of a TiO.sub.2 pigment which has been used widely as a white pigment.
For example, in U.S. Pat. No. 2,242,320, chromium naphthenate is used as a covering material for a surface of the TiO.sub.2 pigment.
In U.S. Pat. No. 2,242,322, in order to improve the resistance against the chalking and discoloring, the TiO.sub.2 pigment is colored by combining 0.5% of chromium in a form of oxides, 2.0% of zirconium silicate and 1.0% of alumina on the calcined TiO.sub.2 pigment.
In U.S. Pat. Nos. 2,226,142 and 2,062,137, chromium compounds are added to the TiO.sub.2 pigment in order to improve the weather resistance before calcining it.
In U.S. Pat. No. 2,045,836, TiO.sub.2 is precipitated in the presence of a chromic acid so as to generate the TiO.sub.2 pigment which includes chromate ions, thereby improving the weather resistance.
In U.S. Pat. No. 2,231,268, a pigment production process is disclosed. The production process includes a preparing step of preparing calcined TiO.sub.2, which includes small amounts of aluminum (from 0.25 to 2% as Al.sub.2 O.sub.3) and chromium (from 0.01 to 2% as Cr.sub.2 O.sub.3), and a drying step.
However, the pearl mica pigment which includes the mica particles coated with the TiO.sub.2 is a more complex substance than the simple TiO.sub.2 pigment is. Hence, the methods or techniques employed for stabilizing the TiO.sub.2 pigment are useless or insufficient to give the stability to the pearl mica pigment. This is because there occurs reactions at the boundaries between the mica and the TiO.sub.2, and it is also because the TiO.sub.2 itself reacts.
Hence, there have been proposed a variety of methods for improving the weather resistance of the pearl mica pigment.
For instance, West Germany Laid-Open Patent Publication No. 1,467,468 sets forth an advantageous effect of chromium hydroxide coating layer which is formed on mica covered with anatase type TiO.sub.2.
West Germany Laid-Open Patent Publication No. 2,852,585 sets forth a similar advantageous effect of chromium hydroxide coating layer for mica which is covered with rutile type TiO.sub.2.
The primary object of these two German patent publications is to obtain a transparent pearl mica pigment.
In Japanese Unexamined Patent Publication (KOKAI) No. 34,527/1972, a method is disclosed in which a pearl mica pigment is treated with methacrylate chromium (III) chloride. When the methacrylate chromium (III) chloride remains without being hydrolyzed substantially on the pearl mica pigment, there arises an advantageous effect of this treatment. The purpose of the treatment is said to give "an excellent resistance against varying conditions" to a coating film comprising this pigment, however, the advantageous effect is available only for the case where the coating film is exposed to moisture.
In Japanese Examined Patent Publication (KOKOKU) No. 3,345/1985, a pearl mica pigment is disclosed, and it is formed as follows. A thin TiO.sub.2 film is precipitated on a mica surface, and thereafter SnO.sub.2 and TiO.sub.2 are coated thereon by turns in this order. After burning the thus coated mica at a temperature up to 1,000.degree. C., it is further coated with an insoluble chromium compound, and it is burned again to form the rutile type TiO.sub.2 on the mica surface.
Even when the conventional pearl mica pigment is mixed with various paints and plastics, it cannot be employed in practical applications because it has an insufficient weather resistance. Therefore, the present inventors carried out the chromium (III) oxide treatments on the outermost surface of the conventional pearl mica pigment in accordance with the above-mentioned related arts, i.e., West Germany Laid-Open Patent Publication No. 1,467,468, West Germany Laid-Open Patent Publication No. 2,852,585, Japanese Unexamined Patent Publication (KOKAI) No. 34,527/1972 and Japanese Examined Patent Publication (KOKOKU) No. 3,345/1985. However, the weather resistance of the conventional pearl mica pigment could not be improved at all.